Just the API and a bare-bone implementation

Requires synchronous and single-threaded, so only 1 channel and 1 outstanding RPC
per channel for now. No open-loop for now

When building the native extension make -j can absorb all available
resources on a system.  Implement "reasonable" limits on the number
of compiling jobs when the number of processors can be detected and
use a conservative count when ruby version does not provide detection.

This reverts commit 5e01e2ac.

This is what `GRPC::Pool::add_rpc_descs_for` expects.

unkown code